Fervid speculation as to Justice Ruth Bader Ginsburg’s well-being was set off in December when the Supreme Court abruptly announced that she was being treated for lung cancer.

Though a positive prognosis has since allayed the worst fears of Ginsburg’s admirers, her continued struggles broach important but little-discussed questions about judicial fitness in an era when unprecedented longevity often entails sudden declines in mental and physical faculties.

Those questions are hardly theoretical: The modern history of the Supreme Court is replete with examples of real or perceived incapacity among the justices, who alone decide when to leave active service.